This invention relates to an improved direct photographic process for preparing a screen structure for a shadow-mask-type cathode-ray tube.
Direct photographic processes for preparing a screen structure for a shadow-mask-type cathode-ray tube have been described previously, for example, in U.S. Pat. Nos. 3,406,068 to H. B. Law and 3,685,994 to H. R. Frey. The tube is usually comprised of a faceplate panel which includes a viewing window and peripheral sidewalls extending from the margins of the window. A mask assembly comprising an apertured mask and temperature-compensating mounting means is mounted in the panel, usually on three or four studs implanted in the panel sidewalls, with the mask spaced a desired distance from the inner surface of the window. Temperature-compensating mounting means for the mask have been described previously, for example, in U.S. Pat. Nos. 3.803,436 to A. M. Morrell and 3,330,980 to T. M. Shrader. The temperature-compensating feature of the mounting means operates to move the mask towards the screen as the tube heats up (to a maximum of about 80.degree.C) during the operation of the tube, so that a projection of electrons through each aperture remains on its associated screen element. Heating causes the mask assembly to expand, moving the off-center apertures outwards from the longitudinal axis of the tube. By moving the mask forward towards the screen, the projection from the off-center apertures upon the screen is moved inward towards the tube axis, thereby compensating for the outward movement caused by heating.
In one method for making a screen structure for a cathode-ray tube having a shadow mask mounted on temperature-compensating mask-mounting means in a faceplate panel, the panel is coated with a layer comprising a light-hardenable material, (with or without phosphor particles), the panel and layer are heated to dry the layer, and the mask is mounted in the panel. Then, actinic light is projected through the mask from a small-area source to expose selected areas of the dry layer to the light so as to harden (insolubilize) the exposed portions of the layer. The exposing step is assumed to take place with all parts of the system at about 22.degree.C. In many situations, the panel is still hot (above 40.degree.C) just prior to and during the exposing step. Heat from the panel warms the temperature-compensating mask-mounting means and causes the mask to move forward towards the layer. This produces off-center exposed areas that are located inwards, resulting in misplaced light-hardened areas, which may later be misregistered with respect to the electron beams impinging on the screen. In some cases, because of the geometry of the mounting means, as with some three-spring structures, the mask assembly is rotated or twisted, as well as screen elements being shifted. Also, a hot panel may cause the mask to dome or become distorted when the mask becomes warm faster than the frame to which it is attached.
The novel method is similar to the prior method except that, after the mask assembly is mounted in the panel, and while the panel is still above 40.degree.C, the temperature-compensating mounting means is cooled to provide a desired spacing between the mask and the panel. The cooling is conducted during the period of the light exposure, and may be commenced before or after the start of the exposure. One convenient method for cooling the mounting means is to pass a stream of air over the mounting means during the exposure. Preferably, the mounting means is cooled with room-temperature air to temperatures below 25.degree.C as desired. Practice of the invention can be used to avoid shifting of screen elements and/or twisting of the mask assembly resulting from the use of a hot panel during the light-exposing step. Additional benefits in dimensional stability of the mask may be achieved in some cases by also cooling the mask itself, as with one or more streams of cooling air. Cooling of the mask can be used to reduce or eliminate doming or anti-doming movements of the mask which may later cause misregister of the electron beams on the elements of the viewing screen.